Transcription And Translation

Advanced Biology Transcription and Translation

RNA differs from DNA

12.1

DNA  Protein An organisms DNA codes for specific traits by dictating the synthesis of proteins The process by which DNA directs protein synthesis, gene expression includes two stages, called transcription and translation

DNA → RNA → protein

12.2

Eukaryotes vs. Prokaryotes

12.3

Codons

12.4

Codons

12.4

Codon Reading Frame AUG is always the “start” codon so all polypeptides begin with Methionine when they are synthesized Having a consistent start codon is necessary so that the reading frame is always the same.

12.5

Transcription The synthesis of RNA from DNA

Promoters and Terminators Are sequences of DNA Promoters signal the initiation of RNA synthesis (TATA) Terminators signal the end of RNA synthesis (AAUAAA in eukaryotes)

12.6

Three Steps

12.7

Alteration of mRNA Ends Each end of a pre-mRNA molecule is modified in a particular way – The 5′ end receives a modified nucleotide cap – The 3′ end gets a poly-A tail TRANSCRIPTION

RNA PROCESSING

DNA

A modified guanine nucleotide added to the 5′ end

Pre-mRNA

mRNA

5′ G P P P

50 to 250 adenine nucleotides added to the 3′ end

Protein-coding segment Polyadenylation signal 3′ AAUAAA

AAA…AAA

Ribosome TRANSLATION Polypeptide

Figure 17.9

12.8

5′ Cap 5′ UTR Start codon Stop codon

3′ UTR

Poly-A tail

Split Genes and RNA Splicing RNA splicing – Removes introns and joins exons TRANSCRIPTION

RNA PROCESSING

DNA Pre-mRNA

5′ Exon Intron Exon Pre-mRNA5′ Cap 30 31 1 Coding segment

mRNA Ribosome

3′ Intron Exon Poly-A tail 104 105 146 Introns cut out and exons spliced together

TRANSLATION

Polypeptide

mRNA 5′ Cap

1 3′ UTR

Figure 17.10

12.8

Poly-A tail 146 3′ UTR

Proteins often have a modular architecture Consisting of discrete structural and functional regions called domains In many cases different exons code for the different domains in a protein DNA

Gene Exon 1

Exon 2

Intron

Intron

Exon 3

Transcription RNA processing Translation Domain 3 Domain 2 Domain 1

12.8

Polypeptide

Translation The RNA directed synthesis of a polypeptide

Translation an overview

tRNA

12.9

Ribosomes

12.10

Initiation

12.11

Elongation

12.11

Termination

12.11

Polyribosomes

12.12

Post Translation Protein structure is determined by amino acid sequence and modifications Modifications include – The attachment of certain sugars, lipids, or phosphate groups – Joining different subunits of the protein to create the quaternary structure

12.13

Point Mutations The change of a single nucleotide in the DNA’s template strand

12.14

Two Types Substitutions – Replacing one or more base with others

Insertions or Deletions – Adding or removing one or more base

12.14